1. Field of the Invention
The present invention relates to a motor drive circuit that controls rotation of a motor including a stator having a plurality of coils and a magnetic rotor, and especially to a motor drive circuit which drives a pulse.
2. Description of the Related Art
A brushless direct-current motor is used for rotating a disc in electronic devices using disc-type media such as portable CDs (Compact Disc), DVDs (Digital Versatile Disc), and the like. In general, the brushless direct-current (DC) motor is provided with a rotor having a permanent magnet, and a stator having a plurality of phase coils connected in a star configuration; the coils are excited by controlling a current supplied to the coils, and the rotor is driven by rotation relative to the stator. In order to detect the current rotation position of the rotor, the brushless DC motor is generally provided with a sensor such as a Hall element, an optical encoder, or the like, and the current supplying each phase coil is switched in accordance with the position detected by the sensor to give an appropriate torque to the rotor.
In order to downsize a motor, a sensorless motor has been proposed, which detects the rotation position of the rotor without using a sensor, such as a Hall element or the like (for example, refer to Patent Document 1). By measuring electrical potential at the midpoint of the wiring of the motor, for example, the sensorless motor detects induced voltage generated in the coils, and the motor and obtains positional information.    Patent Document 1: Japanese Patent Application, Laid Open No. H3-207250    Patent Document 2: Japanese Patent Application, Laid Open No. H10-243685
While driving this type of sensorless motor, in order to reduce noise generated by the motor, it is desirable that the current flowing in each phase coil should gently vary.